The present invention relates to a load cell having strain gauges for converting a mechanical distortion, which is caused by a load, into an electrical signal and measuring the weight of the load.
In general, this type of load cell has two parallel arm portions arranged between the fixing portion and the movable portion to which a load is applied, and includes a distortion-generating body made of a metal wherein two thin deformable portions are formed in each of the arm portions, in positions corresponding to four corners of a parallelogram whose two sides are constituted by those two arms portions. Strain gauges are adhered to each of the thin deformable portions, which, in effect, means they are adhered to both sides of the distortion generating body.
There are, however, several drawbacks inherent in this type of load cell, drawbacks such as complicated manufacturing processes, low yield, and high cost. To eliminate these drawbacks, the strain gauges are formed on one surface of the distortion-generating body, by use of the thin film technique. More precisely, a pair of strain gauges are formed in each of the two thin deformable portions of only one of the two arm portions, these four strain gauges being then bridge-connected.
The structure of such a load cell will now be described in detail, with reference to FIGS. 1 and 2A to 2C. First, distortion-generating body 1 is formed, from a stainless metal high tensile aluminum, or the like. Body 1 includes two parallel extending arm portions 5A and 5B for coupling fixing end portion 2, which is secured to a non-movable portion such as a base or the like, and movable end portion 4, to which is coupled pan 3 (FIGS. 2A to 2C). Arm portions 5A and 5B are formed to have thin deformable portions 6A, 6b, 6C and 6D at positions corresponding to the corners of a parallelogram which is formed by fixing end portion 2, movable end portion 4, and arm portions 5A and 5B. Coupling holes 7 and 8 are formed in fixing end portion 2 and in movable end portion 4, respectively. Using the thin film technique, four strain gauges 9 are formed on one side of distortion generating body 1, through an insulative layer on thin deformable portions 6A and 6B. These strain gauges 9 are bridge-connected by leading portions 10, which are also formed by means of the thin film technique, and are connected to an external circuit by terminal portions 11. In this way, a bridge balance type load cell is formed for generating an output voltage in accordance with a load applied onto pan 3.
In the case where strain gauges 9 are formed on one side of distortion generating body 1, if an unbalanced load acts on distortion generating body 1, various modes of distortion are generated which cannot be cancelled by each other. As a result, the linearity of the load weight-output voltage characteristic is degraded, lowering the accuracy of the load cell.
FIG. 2A shows a state in which a load is applied so that its weight does not act in an unbalanced fashion on distortion generating body 1. In this case, the load weight-output voltage characteristic has the linearity as shown in FIG. 3A. However, in the case of FIGS. 2B and 2C, the load weight-output voltage characteristic has nonlinearity, as is shown in FIGS. 3B and 3C. In FIGS. 3B and 3C, a broken line indicates that the output voltage characteristic may change in the opposite direction when distortion generating body 1 is formed in a different shape.